A solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, generally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
In the fuel cell, in order to supply a fuel gas such as a hydrogen-containing gas and an oxygen-containing gas such as the air to the anode and the cathode of the electrolyte electrode assembly, a fuel gas channel and an oxygen-containing gas channel are formed along surfaces of the separator.
For example, in a flat stack fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2006-120589, as shown in FIG. 17, a separator 1 stacked on a power generation cell is provided. The separator 1 is formed by connecting left and right manifold parts 2a and a part 2b at the center where the power generation cell is provided, by joint parts 2c. The joint parts 2c have flexibility.
The manifold parts 2a has gas holes 3, 4. One gas hole 3 is connected to a fuel gas channel 3a, and the other gas hole 4 is connected to an oxygen-containing gas channel 4a. The fuel gas channel 3a and the oxygen-containing gas channel 4a extend in a spiral pattern into the part 2b, and are opened to a fuel electrode current collector and an air electrode current collector, respectively, at positions near the center of the part 2b. 
In the above conventional technique, the part (sandwiching section) 2b for placing the electrolyte electrode assembly is provided at the center of the separator 1, and across the part 2b the two manifold parts 2a are provided at diagonal positions of the separator 1. In the structure, it is not possible to suitably heat the fuel gas and the oxygen-containing gas flowing through the gas holes 3, 4 by the heat produced in the power generation of the fuel cell before the gases are supplied to the electrolyte electrode assembly. The temperature difference between the fuel gas and the oxygen-containing gas before being supplied to the electrolyte electrode assembly cannot be reduced, and stable power generation in the electrolyte electrode assembly cannot be achieved.
Moreover, the manifold parts 2a where high sealing performance is required are provided separately. In the structure, the sealing pressure tends to be applied to the electrolyte electrode assembly excessively. Under the circumstances, the electrolyte electrode assembly may be damaged undesirably, and the efficient power generation and current collection may not be achieved.
Further, in the separator 1, the joint parts 2c are provided around the part 2b for placing the electrolyte electrode assembly, i.e., around the power generation area. In the structure, it becomes difficult to smoothly discharge the exhaust gas after power generation from the outer circumferential portion of the electrolyte electrode assembly.
Further, the channels of the fuel gas and the oxygen-containing gas extending from the respective manifold parts 2a to the power generation area are not straight. Therefore, the fuel gas and the oxygen-containing gas are not smoothly and efficiently supplied to the power generation area.